A wind turbine can include a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
Some wind turbine configurations include double-fed induction generators (DFIGs). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency (e.g., 50 Hz, 60 Hz, etc.). Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Alternatively, some wind turbine configurations include, but are not limited to, alternative types of induction generators, permanent magnet (PM) synchronous generators and electrically-excited synchronous generators and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
Wind turbines have a plurality of electrical and mechanical components. Each component may have independent or different operating limitations, such as current, voltage, power, and/or temperature limits, than other components. The wind turbine system may also have its own operating limits. The operating limits may be designed or selected to allow the wind turbine to operate at a peak efficiency, output power with desired parameters, and/or to prevent conditions from occurring at the wind turbine which may damage wind turbine components.
As one example, a maximum current limit can be set for the wind turbine. The wind turbine control system can maintain a current of the wind turbine at or below the maximum current limit to protect against overcurrent situations in the generator, the converter, or other turbine system components. The current of the wind turbine can have a real current component and a reactive current component, where real current is the component of current in phase with voltage and reactive current is the component ninety degrees out of phase with voltage.
As another example, a maximum reactive current limit can be set for the wind turbine. The maximum reactive current limit can be a function of the maximum current limit and one or more feedback signals indicative of the real current occurring at the wind turbine. For example, a limiting function can be applied to the maximum current limit and the feedback signals indicative of the real current to determine the maximum reactive current limit. Thus, as the real current fluctuates during operation of the wind turbine, the maximum reactive current limit can be recalculated. The wind turbine control system can maintain a reactive current setpoint at or below the maximum reactive current limit so that the current is maintained at or below the maximum current limit.
Certain existing schemes for calculating the maximum reactive current limit fail to account for imbalances among the three phases of power occurring at the grid to which the wind turbine is coupled. Such imbalances can include, for example, an imbalance in the voltage and/or current among the three phases of grid power. The control system's failure to account for imbalances in the three phases of grid power can result in excessively large current (e.g., in the turbine converter and/or generator). In particular, although the average current magnitude is held to the desired limit value, an individual phase could have excessively high current.